There is known a type of damping valve, called a variable damping valve, capable of changing a damping force of a shock absorber interposed between a chassis and an axle of a vehicle. Such a type of damping valve includes, for example, an annular valve seat provided in the middle of a flow path connected from a cylinder of the shock absorber to a reservoir, a valve body seated on or unseated from the annular valve seat to open or close the flow path, a pilot passage branching from the flow path, an orifice provided in the middle of the pilot passage, a back-pressure chamber provided in the rear side of the valve body oppositely to the valve seat, a pilot valve provided downstream of the pilot passage, and a solenoid for adjusting a valve opening pressure of the pilot valve. In the variable damping valve, a secondary pressure downstream from the orifice in the pilot passage is introduced into the back-pressure chamber to press the valve body.
In the damping valve described above, since the pilot valve is provided downstream from the back-pressure chamber, the secondary pressure guided to the back-pressure chamber is controlled by the valve opening pressure of the pilot valve by adjusting the valve opening pressure of the pilot valve using a thrust force of the solenoid.
As described above, the secondary pressure is applied to the rear face of the valve body, so that the valve body is pressed to the valve seat side. A pressure upstream of the flow path is applied to the front face of the valve body by flexing the valve body to unseat it from the valve seat. Therefore, if the force of unseating the valve body from the valve seat caused by the pressure upstream of the flow path exceeds the force of pressing the valve body to the valve seat caused by the secondary pressure, the valve body is opened.
That is, it is possible to adjust the valve opening pressure of the valve body by controlling the secondary pressure. In addition, it is possible to change resistance applied from the damping valve to a flow of the hydraulic oil passing through the flow path by adjusting the valve opening pressure of the pilot valve using the solenoid. Therefore, it is possible to generate a desired damping force in the shock absorber (for example, see JP 2009-222136 A).